1. Field of the invention
The present invention relates to a method of manufacturing a semiconductor device in which a semiconductor element and a substrate are bonded to each other, and the semiconductor device. In more detail, the invention relates to a method of manufacturing a semiconductor device in which a semiconductor element having an Au—Sn solder is bonded to a substrate by using an Ag paste, and the semiconductor device.
2. Description of the related art
In recent years, a solder containing Pb to be an environmentally hazardous substance is not used, that is, a so-called Pb free solder is used. Among the Pb free solders, an Au—Sn solder having a high thermal conductivity is often used (for example, see Patent Documents 1 and 2).
Since the Au—Sn solder has a poor wettability, however, a sufficiently large bonding region cannot be taken. Since a large number of oxide films are provided on a surface, moreover, a molten Au—Sn solder alloy has a poor flowability. For this reason, there is a process for applying a load to carry out bonding while performing heating and melting. However, a heating temperature is high, and furthermore, a long time is required for heating. In addition, there is a problem in that the load is to be applied for a long period of time in order to carry out the bonding.
Therefore, there is proposed a technique for solving the problems in the use of the Au—Sn solder by utilizing an additional metal such as Ag as well as the Au—Sn solder (for example, see Patent Documents 3 to 6).    Patent Document 1: JP-A-2006-269751 Publication    Patent Document 2: JP-A-2009-302413 Publication    Patent Document 3: JP-A-06-188551 Publication    Patent Document 4: JP-A-2004-072048 Publication    Patent Document 5: JP-A-2007-273744 Publication    Patent Document 6: JP-A-2010-161252 Publication
When Ag is used as the additional metal, however, there is a fear that a problem of a migration might be caused. Moreover, there is also a problem in that an intermetallic compound such as an AuSn2 alloy or an AuSn4 alloy which has a large Sn content and tends to cause a brittle fracture is generated after an Au—Sn solder is molten, resulting in a deterioration in a bonding reliability.